1. Field of the Invention
The present invention relates to a zoom lens system for a compact camera (lens-shutter camera).
2. Description of the Prior Art
In a zoom lens system of a compact camera, a long back focal distance is not necessary, unlike a zoom lens system for a single lens reflex camera (SLR camera) which requires a space behind the photographing lens system for providing a quick-return mirror. Accordingly, in a compact camera, a telephoto lens system, including either a positive lens group and a negative lens group, or a positive lens group, a positive lens group and a negative lens group, in this order from the object, is generally employed. On the other hand, in an SLR camera, a retrofocus lens system, including a negative lens group and a positive lens group, in this order from the object, is generally employed.
Recently, in zoom lens systems for compact cameras, there have been increasing requirements for miniaturization, a higher zoom ratio, and a shorter focal length at the short focal length extremity. In order to satisfy the requirement for miniaturization, it is preferable to utilize a two-lens-group zoom lens system including a small number of lens elements. However, in the two-lens-group zoom lens systems of the prior art, if an attempt is made to obtain the zoom ratio of 3.5 or more, it is difficult to attain an adequate balance of aberrations from the short focal length extremity to the long focal length extremity. Furthermore, if an attempt is made to obtain a shorter focal length at the short focal length extremity, the diameter of the front lens group generally becomes larger, which makes miniaturization of the zoom lens system difficult, and makes the correcting of aberrations difficult. Due to these reasons, it has been difficult, in the prior art, to provide a two-lens-group zoom lens system which can attain miniaturization, a higher zoom ratio, and a shorter focal length at the short focal length extremity at the same time.
An object of the present invention is to provide a small telephoto two-lens-group zoom lens system having a zoom ratio of 3.5 or more, and the half angle-of-view of 35 degrees or more at the short focal length extremity.
In order to achieve the above-mentioned object, there is provided a zoom lens system including a positive first lens group, and a negative second lens, in this order from the object. Zooming is performed by varying the distance between the first lens group and the second lens group. The first lens group includes a negative first sub lens group (1a lens group) and a positive second sub lens group (1b lens group), in this order from the object. The zoom lens system satisfies the following conditions:
5.0 less than ft/f1 less than 7.0xe2x80x83xe2x80x83(1) 
0.9 less than |fw/f1a| less than 1.8xe2x80x83xe2x80x83(2) 
wherein
ft designates the focal length of the entire zoom lens system at the long focal length extremity;
f1 designates the focal length of the first lens group;
fw designates the focal length of the entire zoom lens system at the short focal length extremity; and
f1a designates the focal length of the first sub lens group (1a lens group) of the first lens group.
The most object-side lens element of the first lens group can be a negative lens element having a concave surface facing the object, and preferably satisfies the following condition:
0.1 less than |r1/ft51  less than 0.2xe2x80x83xe2x80x83(3) 
wherein
r1 designates the radius of curvature of the object-side concave surface of the most object-side negative lens element of the first lens group.
It is preferable that at least one surface of lens elements in the first lens group is made aspherical. Similarly, it is preferable that at least one surface of lens elements in the second lens group is made aspherical.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2000-301732 (filed on Oct. 2, 2000) which is expressly incorporated herein in its entirety.